Group by count of objects within an array in Vanilla Javascript

Question

I have an array of objects:

[{person:101, year: 2012}, {person:102, year: 2012}, {person:103, year: 2013}]

And I want to be able to return an

Answer 1

Use a generic group by key reducer that will count the number of items in each group. I will take inspiration from a previous answer of mine. This function will return another function that act as a reducer, but you can always give it the key that you want as a parameter.

const groupByCounter = key => (result,current) => {
  const item = Object.assign({},current);
  if (typeof result[current[key]] == 'undefined'){
    result[current[key]] = 1;
  }else{
    result[current[key]]++;
  }
  return result;
};

const data = [{person:101, year: 2012}, {person:102, year: 2012}, {person:103, year: 2013}];

const group = data.reduce(groupByCounter('year'),{});
console.log(group);